Oil sands, tar sands, or bituminous sands are common names of geological formations that contain bitumen, an extremely heavy type of crude oil. Oil sands can have a variety of compositions but typically include, in addition to the bitumen, water and mineral solids. The mineral solids can include coal and inorganic solids such as coal, sand, and clay. Significant deposits of oil sands can be found in North America. One of the largest oil sands deposits is in the Athabasca region of Alberta, Canada. In the Athabasca region, the oil sands formation can be found at the surface, although it may be buried as deep as two thousand feet below the surface overburden. The oil sands deposits are measured in barrels equivalent of oil. It is estimated that the Athabasca oil sands deposit contains the equivalent of about 1.7 to 2.3 trillion barrels of oil. Global oil sands deposits have been estimated to contain up to 4 trillion barrels of oil. By way of comparison, the proven worldwide oil reserve is estimated to be about 1.3 trillion barrels.
The bitumen, content of oil sands varies from approximately 5 wt. % to 21 wt. %, with a typical content of approximately 12 wt. %. Oil sands also include approximately 1 wt. % to 10 wt. % water. The remainder is mineral matter such as coal, sand, and clay. Bitumen is best described as a thick, sticky form of crude oil that is so heavy and viscous that it will not flow unless heated or diluted with lighter hydrocarbons. At room temperature, bitumen is much like cold molasses.
In the past, bitumen has been extracted from oil sands using a number of technologies. Typical oil sands extraction processes can be divided into two categories: in-situ processes and mining processes. The in-situ processes don't require removal of the oil sands to a processing facility. Instead, bitumen is extracted directly from the oil sands. Typical in-situ processes involve heating the oil sands by injecting steam or in some other suitable manner and then pumping the bitumen out like conventional crude oil.
Mining processes, require excavation and removal of the oil, sands to a processing facility where the bitumen is extracted. For example, a typical mining process may include excavating the oil sands and mixing them with heated water and, optionally, a process aid such as caustic soda (NaOH) which is then piped as a slurry to the extraction plant. Alternatively, the oil sands may be trucked to the extraction plant where the ore is mixed with heated water and/or one or more process aids. Once at the plant, the mixture is agitated to form a bitumen enriched froth. The combination of hot water and agitation releases bitumen from the tar sand, and allows small air bubbles to attach to the bitumen droplets. The bitumen froth floats to the top of separation vessels and is separated for further processing. The bulk of the mineral solids are removed from the bottom of the separation vessels for further processing or disposal. In some processes, middlings may also be removed from a mid-portion of the separation vessels for further processing to isolate bitumen.
The bitumen froth is treated further to remove air from the froth and, to separate the bitumen from residual mineral solids, water, etc. The air may be removed by heating the froth. The bitumen product may be separated from the froth using the counter-current decantation (CCD) process described in the '709 patent, or alternatively with centrifuges. A hydrocarbon solvent is typically added to modify the viscosity of the bitumen and to otherwise facilitate separation of the bitumen. Separation of the froth yields a bitumen product that can be refined similarly to conventional crude oil and tailings that include mineral solids, water, solvent, precipitated asphaltenes, and some residual bitumen.
The tailings, undergo further processing to separate as much solvent as is feasible. The recovered solvent can either be recycled back to the process or otherwise disposed. As an example, the tailings (i.e., the underflow from the CCD circuit) from a process such as the one described in the '709 patent can include 0.5 wt. % to 10 wt. % solvent, or, more likely, 0.75 wt. % to 2 wt. %. The amount of solvent in the tailings may represent approximately 2 wt. % to 10 wt. % of the total solvent used to separate the bitumen. It is desirable to separate as much solvent as possible in order to increase the economics of the process and to meet environmental regulations governing the disposal of the tailings. It should be noted in this regard, that the solvent in the tailings includes free solvent that is not chemically or physically bound to any other component and bound solvent that is chemically and/or physically bound to other components such as asphaltenes. Therefore, it should be recognized that it is desirable to recover as much of the remaining solvent as possible even while recognizing that some amount of solvent will remain bound to and discharged with the asphaltenes.
Current techniques for processing the tailings suffer from a number of deficiencies. For example, in many situations, the tailings are initially separated to remove some of the mineral solids. Unfortunately, these separation operations do not remove as much of the mineral solids as would be desirable. The mineral solids that remain in the water and solvent mixture can adversely impact downstream unit operations such as steam stripping units used to recover the solvent. The abrasive mineral solids in the tailings often lead to extremely high component wear rates of the steam stripping unit, which results in frequent maintenance and high operating costs.
The use of steam stripping to recover the solvent presents a host of other problems. Steam stripping is very energy intensive and expensive to operate. The steam stripping process may account for as much as 5% to 40% of the total operation extraction operating costs. Much of this expense arises from heating the tailings. Another problem associated with conventional tailings solvent recovery units is that they may not fully separate the solvent resulting in significant amounts of solvent being lost. In some situations, there is so much solvent left in the tailings that the tailings pond is at risk of catching fire. Another potential problem is that after being stripped, the asphaltenes may readily reabsorb the solvent. It would be desirable to increase the solvent recovery rate and thereby reduce the amount of solvent in the final disposed tailings.
Another source of problems is the presence of precipitated asphaltenes in the tailings. Asphaltenes are high molecular weight hydrocarbons having a chemical structure that can include stacks of condensed aromatic rings. Due to their high molecular weight, asphaltenes can be found within the least volatile fraction of the bitumen. The problems associated with precipitated asphaltenes can be overcome by using a solvent that does not appreciably precipitate the asphaltenes such as naphtha. However, this presents it own set of problems due to the difficulty of separating naphtha from the tailings. It is desirable to use a more volatile hydrocarbon solvent so that separation of the solvent is easier and less energy intensive.